1721 & 8141 router with WIC 1 56K DSU

I have configured many 1602 and 1721 routers and never ran into this problem before. This problem has now happened on two new routers. A 1721 and a 8141 software Ver 12.4 (16).

Using a 56 K wic 1 dsu connected to a frame relay I am able to ping and telnet to the Ip address on the frame side of the router. The frame side address is 10.3.0.44 255.255.255.0! The problem is that I am unable to ping the FastEthernet side of the router without a laptop or other ethernet device connected to the router. The FastEthernet IP address is 10.4.17.1 255.255.255.0! The FastEthernet sh Interfaces cmd shows: the FastEthernet is up but the line protocol is down until something is pluged in. Only then will the line protocol come up and I will then be able to ping the FastEthernet side of the router.

Can someone tell me what I'm doing wrong! I have always been able to ping the router on the FastEthernet side before without equipment attached.was there a change in the 12.4(16) software or have I left some thing out of the config file????

Replies

What you are describing has been standard behavior of IOS for a very long time - if there is not anything plugged in to an Ethernet interface of the router then the Ethernet interface will show as interface is up line protocol is down. The interface comes to up and up when something is plugged in to the interface.

If you want the interface to be up and up even with nothing plugged in then you can add the command no keepalive under the Ethernet interface.

I believe that it would be best to remove the command after some device is connected to the interface.

To put the question into prespective lets think about what the command does and the implications of disabling the interface keepalives. The interface does keepalives as a sort of mechanism to validate whether it is capable of forwarding packets out the interface. If the line protocol is up, then it indicates that keepalives are working and that the interface should be able to forward packets sent to its subnet. If the interface is protocol down then it indicates that it is not able to forward packets sent to its subnet. For Ethernet interfaces when nothing is plugged in then keepalives will fail and the interface will be protocol down.

One implication of interface being protocol down is that you can not ping the interface (which is what you discovered). When you configure the interface to disable keepalives it will go into line protocol up, and it will respond to pings, and it will signal that it can forward packets. So during the period that keepalives are disabled you have created the possibility of a black hole. The router is indicating that it can forward packets out the interface but in fact the packets will drop because there is nothing to forward them to. So enabling keepalives will help the router to accurately report its forwarding status.